There is a global need for an intensive source of energy which has security of supply, does not pollute and does not add to greenhouse gases. Such energy sources comprise hot gases and liquids which may be used in processes for providing useful outputs. These include industrial or domestic heat for direct heating of materials as in metal extraction or in chemical reactions or heating for buildings. These energy sources may also be transformed into kinetic energy through engines and the generation of electricity, which may be more convenient for some applications such as motors and vital for others such as lighting and electronic apparatus.
Intensive sources of energy currently in use are derived from fossil fuels or nuclear power stations. Fossil fuels are not always found where they are needed, so that they require transportation in bulk around the globe. They produce energy by combustion which always makes greenhouse gases because it is a chemical reaction. Chemical reactions occur at the level of electrons in atoms. By contrast nuclear power depends on the re-arrangement of atomic nuclei with the release of heat. There are essentially two main types of reaction: fission of large unstable nuclei, and fusion of light nuclei. Nuclear fission depends on critical masses of dangerously radioactive fissionable material. As a result it is inherently suited only to the generation of electricity in massive installations. The chain reactions involved require elaborate, expensive control systems to prevent run-away reactions resulting in a meltdown. The entire installation is contaminated with radioactivity, some of which may last for generations and even millennia. There are no known ways of accelerating the process of radioactive decay.
More satisfactory in principle than nuclear fission is nuclear fusion, in which nuclei of light elements such as hydrogen are caused to fuse to form larger stable nuclei such as those of helium. The fusion reaction releases vastly more energy than fission, the input materials are abundant and the products are potentially harmless. However, extremely high temperatures are necessary for fusion to occur, and it may even be necessary to use nuclear fission explosions to obtain enough heat to initiate the process of nuclear fusion. Even if conditions are reached which may be suitable for fusion, say in a plasma, the temperatures are so high that the reactor or torus may be badly damaged. To extract heat, it is necessary to keep the reactants from direct contact with the walls of the vessel by elaborate engineering.
Nuclear fusion occurs naturally in the Sun and heat is produced predominantly by the conversion of hydrogen to helium, which gives a present composition of 74% hydrogen, 25% helium by mass and only the smallest traces of higher elements. Helium has a nucleus so stable that it is known as an alpha-particle. Heat is generated on the formation of the helium nucleus as the vibration of the new configuration of nucleons, which is another form of kinetic energy. Energy is also emitted in various forms as light, for example ultraviolet light, X-rays, visible light, infrared, microwaves and radio waves. Energy is transmitted to nearby atoms and nuclei, adding to their vibration, and increasing their temperature. The very small proportions of elements with higher atomic mass in the Sun shows that these elements are much more difficult to produce. They are thought to originate in supernova explosions which reach very much higher temperatures than the Sun, far too high to reach in an apparatus on Earth. The fusion of hydrogen nuclei to form helium nuclei is essentially the product of collision at high velocities, which is another way of describing temperature. High pressures in the Sun force particles close enough for any resulting vibration to be transmitted to the bulk of the gas, i.e., spreads the heat.
The formation of a helium nucleus from protons is a slow process requiring multiple collisions and astronomical times, so that it is less likely that it could be reproduced economically. There is, however, the possibility of producing helium from the collision of two deuterium nuclei, which occurs naturally. This would be a faster process and yield a greater output of energy, because half the work of fusion is already done. However, the nature of the process is that only one collision is possible, because a hot helium nucleus would not be able to absorb any other nuclei to continue fusion. It would become a hot helium atom and lose its kinetic energy by warming up surrounding gases and the wall of the reactor. The other candidate, tritium, does not occur naturally, and the collision of two tritium nuclei would also be a single event, as would be the collision of, say, a deuterium nucleus with a tritium nucleus. Moreover, the stoichiometry of the reactions suggests the possibility of forming radioactive by-products. However, the introduction of a few tens of grammes of tritium into the plasma of a torus has been shown to produce a significant temperature rise.